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			<h1>Java 集合框架</h1>
<p>
早在 Java 2 中之前，Java 就提供了特设类。比如：Dictionary, Vector, Stack, 和 Properties 这些类用来存储和操作对象组。
</p>
<p>
虽然这些类都非常有用，但是它们缺少一个核心的，统一的主题。由于这个原因，使用 Vector 类的方式和使用 Properties 类的方式有着很大不同。</p>
<p>
集合框架被设计成要满足以下几个目标。</p>
<ul>
	<li>
		<p>该框架必须是高性能的。基本集合（动态数组，链表，树，哈希表）的实现也必须是高效的。</p></li>
	<li>
		<p>该框架允许不同类型的集合，以类似的方式工作，具有高度的互操作性。</p></li>
	<li>
		<p>对一个集合的扩展和适应必须是简单的。</p></li>
</ul>
<p>
为此，整个集合框架就围绕一组标准接口而设计。你可以直接使用这些接口的标准实现，诸如： <strong>LinkedList</strong>, <strong>HashSet</strong>, 和 <strong>TreeSet</strong> 等,除此之外你也可以通过这些接口实现自己的集合。</p>
<p><img decoding="async" src="//www.runoob.com/wp-content/uploads/2014/01/2243690-9cd9c896e0d512ed.gif"></p>
<p>从上面的集合框架图可以看到，Java 集合框架主要包括两种类型的容器，一种是集合（Collection），存储一个元素集合，另一种是图（Map），存储键/值对映射。Collection 接口又有 3 种子类型，List、Set 和 Queue，再下面是一些抽象类，最后是具体实现类，常用的有 <a href="../java/java-arraylist.html" target="_blank" rel="noopener noreferrer">ArrayList</a>、<a href="../java/java-linkedlist.html" rel="noopener noreferrer" target="_blank">LinkedList</a>、<a href="../java/java-hashset.html" rel="noopener noreferrer" target="_blank">HashSet</a>、LinkedHashSet、<a href="../java/java-hashmap.html" rel="noopener noreferrer" target="_blank">HashMap</a>、LinkedHashMap 等等。</p>

<p>
集合框架是一个用来代表和操纵集合的统一架构。所有的集合框架都包含如下内容：</p>

<ul>
<li>
<strong><p>接口：</strong>是代表集合的抽象数据类型。例如 Collection、List、Set、Map 等。之所以定义多个接口，是为了以不同的方式操作集合对象</p></li>
	<li>
		<p><strong>实现（类）：</strong>是集合接口的具体实现。从本质上讲，它们是可重复使用的数据结构，例如：ArrayList、LinkedList、HashSet、HashMap。</p></li>
	<li>
		<p><strong>算法：</strong>是实现集合接口的对象里的方法执行的一些有用的计算，例如：搜索和排序。这些算法被称为多态，那是因为相同的方法可以在相似的接口上有着不同的实现。</p></li>
</ul><p>
除了集合，该框架也定义了几个 Map 接口和类。Map 里存储的是键/值对。尽管 Map 不是集合，但是它们完全整合在集合中。</p>
<h3>集合框架体系如图所示</h3>
<p><img decoding="async" src="https://www.runoob.com/wp-content/uploads/2014/01/java-coll-2020-11-16.png"></p>
<p>Java 集合框架提供了一套性能优良，使用方便的接口和类，java集合框架位于java.util包中， 所以当使用集合框架的时候需要进行导包。</p>
<hr><h2>集合接口</h2>

<p>
集合框架定义了一些接口。本节提供了每个接口的概述：</p>

<table class="reference">
<tbody>
<tr>
<th width="10%">序号</th>
<th>接口描述</th>
</tr>
<tr>
<td>1</td>
<td>Collection 接口<br><p>
Collection 是最基本的集合接口，一个 Collection 代表一组 Object，即 Collection 的元素, Java不提供直接继承自Collection的类，只提供继承于的子接口(如List和set)。</p>
<p>Collection 接口存储一组不唯一，无序的对象。</p>
</td>
</tr>
<tr>
<td>2</td>
<td> List 接口<br><p>
List接口是一个有序的 Collection，使用此接口能够精确的控制每个元素插入的位置，能够通过索引(元素在List中位置，类似于数组的下标)来访问List中的元素，第一个元素的索引为 0，而且允许有相同的元素。</p>
<p>List 接口存储一组不唯一，有序（插入顺序）的对象。</p>
</td>
</tr>
<tr>
<td>3</td>
<td> Set<br>
<p>
Set 具有与 Collection 完全一样的接口，只是行为上不同，Set 不保存重复的元素。</p>
<p>Set 接口存储一组唯一，无序的对象。</p>
</td>
</tr>
<tr>
<td>4</td>
<td> SortedSet <br>

继承于Set保存有序的集合。</td>
</tr>
<tr>
<td>5</td>
<td> Map<br>


<p>Map 接口存储一组键值对象，提供key（键）到value（值）的映射。</p>
</td>
</tr>
<tr>
<td>6</td>
<td> Map.Entry <br>

描述在一个Map中的一个元素（键/值对）。是一个 Map 的内部接口。</td>
</tr>
<tr>
<td>7</td>
<td> SortedMap<br>

继承于 Map，使 Key 保持在升序排列。</td>
</tr>
<tr>
<td>8</td>
<td> Enumeration<br>

这是一个传统的接口和定义的方法，通过它可以枚举（一次获得一个）对象集合中的元素。这个传统接口已被迭代器取代。</td>
</tr>
</tbody>
</table>
<h3>Set和List的区别</h3>
<ul>
<li>
<p>
1. Set 接口实例存储的是无序的，不重复的数据。List 接口实例存储的是有序的，可以重复的元素。</p></li><li><p>
2. Set 检索效率低下，删除和插入效率高，插入和删除不会引起元素位置改变 <strong>&lt;实现类有HashSet,TreeSet&gt;</strong>。</p></li><li><p>
3. List 和数组类似，可以动态增长，根据实际存储的数据的长度自动增长 List 的长度。查找元素效率高，插入删除效率低，因为会引起其他元素位置改变 <strong>&lt;实现类有ArrayList,LinkedList,Vector&gt;</strong> 。</p></li></ul>

</ul>
<hr><h2>集合实现类（集合类）</h2>
<p>
Java提供了一套实现了Collection接口的标准集合类。其中一些是具体类，这些类可以直接拿来使用，而另外一些是抽象类，提供了接口的部分实现。
</p><p>标准集合类汇总于下表：</p>
<table class="reference">
<tbody>
<tr>
<th width="10%">序号</th>
<th>类描述</th>
</tr>
<tr>
<td>1</td>
<td><strong>AbstractCollection </strong><br>

实现了大部分的集合接口。</td>
</tr>
<tr>
<td>2</td>
<td><strong>AbstractList </strong><br>

继承于AbstractCollection 并且实现了大部分List接口。</td>
</tr>
<tr>
<td>3</td>
<td><strong>AbstractSequentialList </strong><br>

继承于 AbstractList ，提供了对数据元素的链式访问而不是随机访问。</td>
</tr>
<tr>
<td>4</td>
<td><a href="../java/java-linkedlist.html" rel="noopener noreferrer" target="_blank">LinkedList</a><br>
<p>
该类实现了List接口，允许有null（空）元素。主要用于创建链表数据结构，该类没有同步方法，如果多个线程同时访问一个List，则必须自己实现访问同步，解决方法就是在创建List时候构造一个同步的List。例如：  
</p>
<pre>
List list=Collections.synchronizedList(newLinkedList(...));
</pre>
<p>LinkedList 查找效率低。</p>
</td>
</tr>
<tr>
<td>5</td>
<td><a href="../java/java-arraylist.html" target="_blank" rel="noopener noreferrer">ArrayList</a><br>
<p>
该类也是实现了List的接口，实现了可变大小的数组，随机访问和遍历元素时，提供更好的性能。该类也是非同步的,在多线程的情况下不要使用。ArrayList 增长当前长度的50%，插入删除效率低。
</p></td>
</tr>
<tr>
<td>6</td>
<td><strong>AbstractSet </strong><br>

继承于AbstractCollection 并且实现了大部分Set接口。</td>
</tr>
<tr>
<td>7</td>
<td><a href="../java/java-hashset.html" rel="noopener noreferrer" target="_blank">HashSet</a><br><p>
该类实现了Set接口，不允许出现重复元素，不保证集合中元素的顺序，允许包含值为null的元素，但最多只能一个。</p></td>
</tr>
<tr>
<td>8</td>
<td>LinkedHashSet<br>

具有可预知迭代顺序的 <tt>Set</tt> 接口的哈希表和链接列表实现。</td>
</tr>
<tr>
<td>9</td>
<td>TreeSet<br>
<p>该类实现了Set接口，可以实现排序等功能。</p></td>
</tr>
<tr>
<td>10</td>
<td><strong>AbstractMap </strong><br>

实现了大部分的Map接口。</td>
</tr>
<tr>
<td>11</td>
<td><a href="../java/java-hashmap.html" rel="noopener noreferrer" target="_blank">HashMap</a>
<br>
HashMap 是一个散列表，它存储的内容是键值对(key-value)映射。<br>

该类实现了Map接口，根据键的HashCode值存储数据，具有很快的访问速度，最多允许一条记录的键为null，不支持线程同步。
</td>
</tr>
<tr>
<td>12</td>
<td>TreeMap
<br>
继承了AbstractMap，并且使用一颗树。</td>
</tr>
<tr>
<td>13</td>
<td>WeakHashMap
<br>
继承AbstractMap类，使用弱密钥的哈希表。</td>
</tr>
<tr>
<td>14</td>
<td>LinkedHashMap
<br>
继承于HashMap，使用元素的自然顺序对元素进行排序.</td>
</tr>
<tr>
<td>15</td>
<td>IdentityHashMap
<br>
继承AbstractMap类，比较文档时使用引用相等。</td>
</tr>
</tbody>
</table>



<p>在前面的教程中已经讨论通过java.util包中定义的类，如下所示：
</p>
<table class="reference">
<tbody>
<tr>
<th width="10%">序号</th>
<th>类描述</th>
</tr>
<tr>
<td>1</td>
<td>Vector
<br>
<p>该类和ArrayList非常相似，但是该类是同步的，可以用在多线程的情况，该类允许设置默认的增长长度，默认扩容方式为原来的2倍。</p>
</td>
</tr>
<tr>
<td>2</td>
<td>Stack
<br>
栈是Vector的一个子类，它实现了一个标准的后进先出的栈。</td>
</tr>
<tr>
<td>3</td>
<td>Dictionary
<br>
Dictionary 类是一个抽象类，用来存储键/值对，作用和Map类相似。</td>
</tr>
<tr>
<td>4</td>
<td>Hashtable
<br><p>
Hashtable 是 Dictionary(字典) 类的子类，位于 java.util 包中。</p></td>
</tr>
<tr>
<td>5</td>
<td>Properties
<br>
Properties 继承于 Hashtable，表示一个持久的属性集，属性列表中每个键及其对应值都是一个字符串。</td>
</tr>
<tr>
<td>6</td>
<td>BitSet<br>
一个Bitset类创建一种特殊类型的数组来保存位值。BitSet中数组大小会随需要增加。</td>
</tr>
</tbody>
</table>

<hr>
<h2>集合算法</h2>
<p>
集合框架定义了几种算法，可用于集合和映射。这些算法被定义为集合类的静态方法。</p>
<p>
在尝试比较不兼容的类型时，一些方法能够抛出 ClassCastException异常。当试图修改一个不可修改的集合时，抛出UnsupportedOperationException异常。</p>
<p>
集合定义三个静态的变量：EMPTY_SET，EMPTY_LIST，EMPTY_MAP的。这些变量都不可改变。</p>
<p>



<table class="reference">
	<tbody>
		<tr>
			<th width="10%">
				序号</th>
			<th>
				算法描述</th>
		</tr>
		<tr>
			<td>
				1</td>
			<td>
				Collection Algorithms<br>
				这里是一个列表中的所有算法实现。</td>
		</tr>
	</tbody>
</table>
<hr>

<h2>
如何使用迭代器
</h2>
<p>
通常情况下，你会希望遍历一个集合中的元素。例如，显示集合中的每个元素。</p>
<p>
一般遍历数组都是采用for循环或者增强for，这两个方法也可以用在集合框架，但是还有一种方法是采用迭代器遍历集合框架，它是一个对象，实现了<a href="../java/java-iterator.html" rel="noopener noreferrer" target="_blank">Iterator</a> 接口或 ListIterator接口。</p>
<p>
迭代器，使你能够通过循环来得到或删除集合的元素。ListIterator 继承了 Iterator，以允许双向遍历列表和修改元素。</p>
<table class="reference">
	<tbody>
		<tr>
			<th width="10%">
				序号</th>
			<th>
				迭代器方法描述</th>
		</tr>
		<tr>
			<td>
				1</td>
			<td>
				<a href="../java/java-iterator.html" rel="noopener noreferrer" target="_blank">使用 Java Iterator</a><br>这里通过实例列出  Iterator 和 ListIterator 接口提供的所有方法。</td>
		</tr>
	</tbody>
</table>
<h3>遍历 ArrayList</h3>
<div class="example"> 
<h2 class="example">实例</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-reserved">import</span><span class="hl-code"> </span><span class="hl-identifier">java</span><span class="hl-code">.</span><span class="hl-identifier">util</span><span class="hl-code">.*;
 
</span><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-reserved">class</span><span class="hl-code"> </span><span class="hl-identifier">Test</span><span class="hl-brackets">{</span><span class="hl-code">
 </span><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-types">static</span><span class="hl-code"> </span><span class="hl-types">void</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-brackets">[</span><span class="hl-brackets">]</span><span class="hl-code"> </span><span class="hl-identifier">args</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
     </span><span class="hl-identifier">List</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">&gt; </span><span class="hl-identifier">list</span><span class="hl-code">=</span><span class="hl-reserved">new</span><span class="hl-code"> </span><span class="hl-identifier">ArrayList</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">&gt;</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-identifier">list</span><span class="hl-code">.</span><span class="hl-identifier">add</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">Hello</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-identifier">list</span><span class="hl-code">.</span><span class="hl-identifier">add</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">World</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-identifier">list</span><span class="hl-code">.</span><span class="hl-identifier">add</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">HAHAHAHA</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-comment">//</span><span class="hl-comment">第一种遍历方法使用 For-Each 遍历 List</span><span class="hl-comment"></span><span class="hl-code">
     </span><span class="hl-reserved">for</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">str</span><span class="hl-code"> : </span><span class="hl-identifier">list</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">            </span><span class="hl-comment">//</span><span class="hl-comment">也可以改写 for(int i=0;i&lt;list.size();i++) 这种形式</span><span class="hl-comment"></span><span class="hl-code">
        </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-identifier">str</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-brackets">}</span><span class="hl-code">
 
     </span><span class="hl-comment">//</span><span class="hl-comment">第二种遍历，把链表变为数组相关的内容进行遍历</span><span class="hl-comment"></span><span class="hl-code">
     </span><span class="hl-identifier">String</span><span class="hl-brackets">[</span><span class="hl-brackets">]</span><span class="hl-code"> </span><span class="hl-identifier">strArray</span><span class="hl-code">=</span><span class="hl-reserved">new</span><span class="hl-code"> </span><span class="hl-identifier">String</span><span class="hl-brackets">[</span><span class="hl-identifier">list</span><span class="hl-code">.</span><span class="hl-identifier">size</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">]</span><span class="hl-code">;
     </span><span class="hl-identifier">list</span><span class="hl-code">.</span><span class="hl-identifier">toArray</span><span class="hl-brackets">(</span><span class="hl-identifier">strArray</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-reserved">for</span><span class="hl-brackets">(</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">i</span><span class="hl-code">=</span><span class="hl-number">0</span><span class="hl-code">;</span><span class="hl-identifier">i</span><span class="hl-code">&lt;</span><span class="hl-identifier">strArray</span><span class="hl-code">.</span><span class="hl-identifier">length</span><span class="hl-code">;</span><span class="hl-identifier">i</span><span class="hl-code">++</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-comment">//</span><span class="hl-comment">这里也可以改写为  for(String str:strArray) 这种形式</span><span class="hl-comment"></span><span class="hl-code">
     </span><span class="hl-brackets">{</span><span class="hl-code">
        </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-identifier">strArray</span><span class="hl-brackets">[</span><span class="hl-identifier">i</span><span class="hl-brackets">]</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-brackets">}</span><span class="hl-code">
     
    </span><span class="hl-comment">//</span><span class="hl-comment">第三种遍历 使用迭代器进行相关遍历</span><span class="hl-comment"></span><span class="hl-code">
     
     </span><span class="hl-identifier">Iterator</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">&gt; </span><span class="hl-identifier">ite</span><span class="hl-code">=</span><span class="hl-identifier">list</span><span class="hl-code">.</span><span class="hl-identifier">iterator</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-reserved">while</span><span class="hl-brackets">(</span><span class="hl-identifier">ite</span><span class="hl-code">.</span><span class="hl-identifier">hasNext</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-comment">//</span><span class="hl-comment">判断下一个元素之后有值</span><span class="hl-comment"></span><span class="hl-code">
     </span><span class="hl-brackets">{</span><span class="hl-code">
         </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-identifier">ite</span><span class="hl-code">.</span><span class="hl-identifier">next</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
     </span><span class="hl-brackets">}</span><span class="hl-code">
 </span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-brackets">}</span></div>
</div><br>
</div>
<p><strong>解析：  </strong>
</p><p>
三种方法都是用来遍历ArrayList集合，第三种方法是采用迭代器的方法，该方法可以不用担心在遍历的过程中会超出集合的长度。
</p>
<h3>遍历 Map</h3>
<div class="example"> 
<h2 class="example">实例</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-reserved">import</span><span class="hl-code"> </span><span class="hl-identifier">java</span><span class="hl-code">.</span><span class="hl-identifier">util</span><span class="hl-code">.*;
 
</span><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-reserved">class</span><span class="hl-code"> </span><span class="hl-identifier">Test</span><span class="hl-brackets">{</span><span class="hl-code">
     </span><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-types">static</span><span class="hl-code"> </span><span class="hl-types">void</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-brackets">[</span><span class="hl-brackets">]</span><span class="hl-code"> </span><span class="hl-identifier">args</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
      </span><span class="hl-identifier">Map</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">, </span><span class="hl-identifier">String</span><span class="hl-code">&gt; </span><span class="hl-identifier">map</span><span class="hl-code"> = </span><span class="hl-reserved">new</span><span class="hl-code"> </span><span class="hl-identifier">HashMap</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">, </span><span class="hl-identifier">String</span><span class="hl-code">&gt;</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">put</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">1</span><span class="hl-quotes">&quot;</span><span class="hl-code">, </span><span class="hl-quotes">&quot;</span><span class="hl-string">value1</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">put</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">2</span><span class="hl-quotes">&quot;</span><span class="hl-code">, </span><span class="hl-quotes">&quot;</span><span class="hl-string">value2</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">put</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">3</span><span class="hl-quotes">&quot;</span><span class="hl-code">, </span><span class="hl-quotes">&quot;</span><span class="hl-string">value3</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
      
      </span><span class="hl-comment">//</span><span class="hl-comment">第一种：普遍使用，二次取值</span><span class="hl-comment"></span><span class="hl-code">
      </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">通过Map.keySet遍历key和value：</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-reserved">for</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">key</span><span class="hl-code"> : </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">keySet</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
       </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">key= </span><span class="hl-quotes">&quot;</span><span class="hl-code">+ </span><span class="hl-identifier">key</span><span class="hl-code"> + </span><span class="hl-quotes">&quot;</span><span class="hl-string"> and value= </span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">get</span><span class="hl-brackets">(</span><span class="hl-identifier">key</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-brackets">}</span><span class="hl-code">
      
      </span><span class="hl-comment">//</span><span class="hl-comment">第二种</span><span class="hl-comment"></span><span class="hl-code">
      </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">通过Map.entrySet使用iterator遍历key和value：</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-identifier">Iterator</span><span class="hl-code">&lt;</span><span class="hl-identifier">Map</span><span class="hl-code">.</span><span class="hl-identifier">Entry</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">, </span><span class="hl-identifier">String</span><span class="hl-code">&gt;&gt; </span><span class="hl-identifier">it</span><span class="hl-code"> = </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">entrySet</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">.</span><span class="hl-identifier">iterator</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-reserved">while</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">it</span><span class="hl-code">.</span><span class="hl-identifier">hasNext</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
       </span><span class="hl-identifier">Map</span><span class="hl-code">.</span><span class="hl-identifier">Entry</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">, </span><span class="hl-identifier">String</span><span class="hl-code">&gt; </span><span class="hl-identifier">entry</span><span class="hl-code"> = </span><span class="hl-identifier">it</span><span class="hl-code">.</span><span class="hl-identifier">next</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">;
       </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">key= </span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">entry</span><span class="hl-code">.</span><span class="hl-identifier">getKey</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code"> + </span><span class="hl-quotes">&quot;</span><span class="hl-string"> and value= </span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">entry</span><span class="hl-code">.</span><span class="hl-identifier">getValue</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-brackets">}</span><span class="hl-code">
      
      </span><span class="hl-comment">//</span><span class="hl-comment">第三种：推荐，尤其是容量大时</span><span class="hl-comment"></span><span class="hl-code">
      </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">通过Map.entrySet遍历key和value</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-reserved">for</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">Map</span><span class="hl-code">.</span><span class="hl-identifier">Entry</span><span class="hl-code">&lt;</span><span class="hl-identifier">String</span><span class="hl-code">, </span><span class="hl-identifier">String</span><span class="hl-code">&gt; </span><span class="hl-identifier">entry</span><span class="hl-code"> : </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">entrySet</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
       </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">key= </span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">entry</span><span class="hl-code">.</span><span class="hl-identifier">getKey</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code"> + </span><span class="hl-quotes">&quot;</span><span class="hl-string"> and value= </span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">entry</span><span class="hl-code">.</span><span class="hl-identifier">getValue</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-brackets">}</span><span class="hl-code">
    
      </span><span class="hl-comment">//</span><span class="hl-comment">第四种</span><span class="hl-comment"></span><span class="hl-code">
      </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">通过Map.values()遍历所有的value，但不能遍历key</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-reserved">for</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">v</span><span class="hl-code"> : </span><span class="hl-identifier">map</span><span class="hl-code">.</span><span class="hl-identifier">values</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
       </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">value= </span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">v</span><span class="hl-brackets">)</span><span class="hl-code">;
      </span><span class="hl-brackets">}</span><span class="hl-code">
     </span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-brackets">}</span></div>
</div><br>
</div>

<hr>
<h2>
如何使用比较器
</h2>
<p>
TreeSet和TreeMap的按照排序顺序来存储元素. 然而，这是通过比较器来精确定义按照什么样的排序顺序。</p>
<p>
这个接口可以让我们以不同的方式来排序一个集合。</p>
<table class="reference">
	<tbody>
		<tr>
			<th width="10%">
				序号</th>
			<th>
				比较器方法描述</th>
		</tr>
		<tr>
			<td>
				1</td>
			<td>
				使用 Java Comparator<br>
				这里通过实例列出Comparator接口提供的所有方法</td>
		</tr>
	</tbody>
</table>
<hr>
<h2>总结
</h2>
<p>
Java集合框架为程序员提供了预先包装的数据结构和算法来操纵他们。</p>
<p>
集合是一个对象，可容纳其他对象的引用。集合接口声明对每一种类型的集合可以执行的操作。</p>
<p>
集合框架的类和接口均在java.util包中。</p>
<p>任何对象加入集合类后，自动转变为Object类型，所以在取出的时候，需要进行强制类型转换。</p>			<!-- 其他扩展 -->
						
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	<ol class="commentlist">
		<li class="comment even thread-even depth-1" id="comment-21701"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;分享</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;Com***able@jihe.com</p></div><div id="runoobvote-id-21701" data-commid = "21701" class="upvotejs"><a class="upvote"></a> <span class="count">141</span></div></div><div class="comt-main" id="div-comment-21701"><p><strong>ArrayList 和 LinkedList 的区别</strong></p>

<p>ArrayList 是 List 接口的一种实现，它是使用数组来实现的。</p>

<p>LinkedList 是 List 接口的一种实现，它是使用链表来实现的。</p>

<p>ArrayList 遍历和查找元素比较快。LinkedList 遍历和查找元素比较慢。</p>

<p>ArrayList 添加、删除元素比较慢。LinkedList 添加、删除元素比较快。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">分享</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;分享</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;Com***able@jihe.com</p></div></span>5年前 (2017-12-15)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-21705"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;分享</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;Com***able@jihe.com</p></div><div id="runoobvote-id-21705" data-commid = "21705" class="upvotejs"><a class="upvote"></a> <span class="count">76</span></div></div><div class="comt-main" id="div-comment-21705"><p><b>任何对象加入集合类后，自动转变为Object类型，所以在取出的时候，需要进行强制类型转换。</b>&nbsp;</p><p></p><p></p><p><b></b>任何对象没有使用泛型之前会自动转换Object类型，使用泛型之后不用强制转换。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">分享</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;分享</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;Com***able@jihe.com</p></div></span>5年前 (2017-12-15)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-23813"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;影法师</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;ren***e1@sina.com</p></div><div id="runoobvote-id-23813" data-commid = "23813" class="upvotejs"><a class="upvote"></a> <span class="count">63</span></div></div><div class="comt-main" id="div-comment-23813"><p>有关于 <span class="marked">map.entrySet()</span> 和 <span class="marked">keySet()</span>:</p>

<p>1、如果遍历 hashMap() 时 entrySet() 方法是将 key 和 value 全部取出来,所以性能开销是可以预计的, 而 keySet() 方法进行遍历的时候是根据取出的 key 值去查询对应的 value 值, 所以如果 key 值是比较简单的结构(如 1,2,3...)的话性能消耗上是比 entrySet() 方法低, 但随着 key 值得复杂度提高 entrySet() 的优势就会显露出来。</p>

<p>2、综合比较在只遍历 key 的时候使用 keySet(), 在只遍历 value 的是使用 values() 方法, 在遍历 key-value 的时候使用 entrySet() 是比较合理的选择。</p>

<p>3、如果遍历 TreeMap 的时候, 不同于 HashMap 在遍历 ThreeMap 的 key-value 时候务必使用 entrySet() 它要远远高于其他两个的性能, 同样只遍历 key 的时候使用 keySet(), 在只遍历 value 的是使用 values() 方法对于 TreeMap 也同样适用。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">影法师</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;影法师</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;ren***e1@sina.com</p></div></span>5年前 (2018-02-02)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-28735"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;°</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;858***854@qq.com</p></div><div id="runoobvote-id-28735" data-commid = "28735" class="upvotejs"><a class="upvote"></a> <span class="count">29</span></div></div><div class="comt-main" id="div-comment-28735"><p>集合框架被设计成要满足以下几个目标。</p><ul><li>该框架必须是高性能的。基本集合（动态数组，链表，树，哈希表）的实现也必须是高效的。</li><li>该框架允许不同类型的集合，以类似的方式工作，具有高度的互操作性。</li><li>对一个集合的扩展和适应必须是简单的。</li></ul>
<p>所有的集合框架都包含如下内容：</p>
<ul><li><strong>接口：</strong>是代表集合的抽象数据类型。例如 Collection、List、Set、Map 等。之所以定义多个接口，是为了以不同的方式操作集合对象</li><li><strong>实现（类）：</strong>是集合接口的具体实现。从本质上讲，它们是可重复使用的数据结构，例如：ArrayList、LinkedList、HashSet、HashMap。</li><li><strong>算法：</strong>是实现集合接口的对象里的方法执行的一些有用的计算，例如：搜索和排序。这些算法被称为多态，那是因为相同的方法可以在相似的接口上有着不同的实现。</li></ul>Set和List的区别<ul><li>1. Set 接口实例存储的是无序的，不重复的数据。List 接口实例存储的是有序的，可以重复的元素。</li><li>2. Set检索效率低下，删除和插入效率高，插入和删除不会引起元素位置改变&nbsp;<strong>&lt;实现类有HashSet,TreeSet&gt;</strong>。</li><li>3. List和数组类似，可以动态增长，根据实际存储的数据的长度自动增长List的长度。查找元素效率高，插入删除效率低，因为会引起其他元素位置改变&nbsp;<strong>&lt;实现类有ArrayList,LinkedList,Vector&gt;</strong>&nbsp;。</li></ul><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">°</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;°</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;858***854@qq.com</p></div></span>5年前 (2018-06-25)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-36410"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;zhangchaohong137</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;251***396@qq.com</p></div><div id="runoobvote-id-36410" data-commid = "36410" class="upvotejs"><a class="upvote"></a> <span class="count">40</span></div></div><div class="comt-main" id="div-comment-36410"><p>集合还有快速失败（fail—fast）和安全失败（fail—safe）机制，想要深入的话得去了解一下（<a href="../w3cnote/java-fail-ast-fail-safe.html" rel="noopener" target="_blank">Java 的快速失败和安全失败</a>），解决方法我在此整理一下吧：</p>

<p>1、在单线程的遍历过程中，如果要进行 remove 操作，可以调用迭代器的 remove 方法而不是集合类的 remove 方法。</p>

<p>2、使用 java 并发包(java.util.concurrent)中的类来代替 ArrayList 和 HashMap。比如使用 CopyOnWriterArrayList 代替 ArrayList，CopyOnWriterArrayList 在是使用上跟 ArrayList 几乎一样，CopyOnWriter 是写时复制的容器(COW)，在读写时是线程安全的。该容器在对 add 和 remove 等操作时，并不是在原数组上进行修改，而是将原数组拷贝一份，在新数组上进行修改，待完成后，才将指向旧数组的引用指向新数组，所以对于 CopyOnWriterArrayList 在迭代过程并不会发生 fail-fast 现象。但 CopyOnWrite 容器只能保证数据的最终一致性，不能保证数据的实时一致性。对于 HashMap，可以使用 ConcurrentHashMap，ConcurrentHashMap 采用了锁机制，是线程安全的。在迭代方面，ConcurrentHashMap 使用了一种不同的迭代方式。在这种迭代方式中，当 iterator 被创建后集合再发生改变就不再是抛出 ConcurrentModificationException，取而代之的是在改变时new新的数据从而不影响原有的数据 ，iterator 完成后再将头指针替换为新的数据 ，这样 iterator 线程可以使用原来老的数据，而写线程也可以并发的完成改变。即迭代不会发生 fail-fast，但不保证获取的是最新的数据。
</p>
<div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">zhangchaohong137</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;zhangchaohong137</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;251***396@qq.com</p></div></span>5年前 (2018-10-30)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-78554"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Carpen</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;pan***peng2016@126.com</p></div><div id="runoobvote-id-78554" data-commid = "78554" class="upvotejs"><a class="upvote"></a> <span class="count">2</span></div></div><div class="comt-main" id="div-comment-78554"><p>这里纠正一下List和Set的区别，Set检索效率是比List高的。</p><p>Set的<strong>检索效率</strong>相对于List来说较高，<strong>其原因在于Set是基于哈希表或树等数据结构来存储元素的。在查找元素时，Set会先计算元素的哈希值或使用比较器进行比较，然后再根据哈希值或比较结果在数据结构中查找元素。这种方式的时间复杂度为O(1)或O(logN)</strong>，因此Set的检索效率相对较高。</p><p>而List的检索效率相对于Set来说较低，其原因在于List是基于数组或链表等数据结构来存储元素的。在查找元素时，List需要依次遍历元素，直到找到目标元素或遍历完所有元素。这种方式的时间复杂度为O(N)，因此List的检索效率相对较低。</p><p>容易弄混的地方在于随机访问元素和检索元素，这两者不是一回事。</p><p><b>随机访问元素</b>是指可以通过索引或偏移量来访问集合中的元素，例如使用 ArrayList 中的 get(int index) 方法。随机访问元素的效率通常非常高，因为它可以直接访问内部数据结构中的元素，时间复杂度为O(1)。</p><p><b>检索元素</b>是指在集合中查找特定元素的过程，例如使用 Set 中的 contains(Object o) 方法。检索元素的效率取决于集合的实现方式和元素数量，通常需要遍历集合中的所有元素来查找目标元素，时间复杂度为O(N)。</p><p>再补充一点，于大规模数据，Set的内存占用较大，而ArrayList的性能较优。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">Carpen</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Carpen</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;pan***peng2016@126.com</p></div></span>3周前 (04-06)</div></div></li><!-- #comment-## -->
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